As is well-known, there are two main types of Stirling cycle thermal machines. These are the double cylinder, two piston type and the single cylinder, piston and displacer type. Each of these types has two working spaces filled with the working fluid and connected by a duct which includes fixed regenerator and heat exchangers therein. The working spaces are at the different extreme temperatures of the working cycle and one space is for expansion of the working fluid or gas while the other space is for compression thereof. The two pistons of the double cylinder type are connected by suitable linkages to a crankshaft at which power input is provided or power output is derived. The crankshaft and linkages maintain a proper phase relationship between the two pistons such that their respective working spaces are appropriately varied in volume approximately in conformance with the Stirling thermodynamic cycle.
Similarly, the piston and displacer of the single cylinder type are connected by suitable linkages to a crankshaft where power input is supplied or power output is delivered. The crankshaft and linkages of the single cylinder type also maintain a proper phase relationship between the piston and displacer such that the expansion and compression spaces respectively at the two ends of the single cylinder are appropriately varied in volume according to the Stirling cycle. The piston alternately compresses and expands the working fluid as the displacer, which separates the working spaces, synchronously shifts the working fluid through the regenerator and heat exchangers back and forth between the connected spaces. The movement of the displacer is timed to place most of the working fluid in the compression space when the piston makes its compression stroke, and most of the fluid in the expansion space during its expansion stroke. The Stirling cycle has the same efficiency as the well-known Carnot cycle for the same operating temperature limits but differs from the latter cycle in that the two adiabatic lines thereof are replaced by two constant volume lines.
The Stirling cycle is a thermodynamic cycle wherein a fluid or gas alternately undergoes constant volume and constant temperature processes and in which the heat-up and cool-down of the gas is done at constant volume by a thermal regenerator. This cycle has Carnot cycle efficiency. The Ericsson cycle is similar to the Stirling cycle except that the heat-up and cool-down of the gas is done at constant pressure by the regenerator. This cycle also has Carnot cycle efficiency.
The real engine with a mechanical linkage that places the two pistons, or the piston and displacer, in simple harmonic motion 90 degrees out of phase with each other rounds the corners of the idealized thermodynamic cycles mentioned above. In the real engine, the heat-up and cool-down of the gas is actually done at changing volume and pressure by a regenerator. Nevertheless, if it assumed that the regenerator is perfect and heat transfer to and from the gas is perfect, then this engine, loosely called a Stirling cycle engine, also has Carnot cycle efficiency.
The original Stirling engine design was starved for heat exchange surface. As the gas moved back and forth it never attained either the heat source or heat sink temperature, so the potential power attainable was not obtained. For many years the original design by Robert Stirling was little improved upon. The regenerator screen was usually removed which decreased the internal flow losses at the expense of decreased heat transfer capability but with a net gain in performance.
Two early developers hit upon the isothermalizer principle to eliminate heat transfer starvation in their engines. One was Napier and Rankine who built in about 1854 an engine in which the heater was part of the hot space and the cooler was part of the cold space. These heat exchangers were bundles of closed end tubes with rods fitting down into each tube to displace the gas. For its time it was a very advanced design. To my knowledge, there is no record of how it worked. In 1874 it was reported that several cooling engines had used nesting cone isothermalizers.
Since 1875 the history of designers who have chosen the isothermalizer tradeoff has been quite sparse. The Newton U.S. Pat. No. 2,803,951 describes refrigerating compressors using a finned cone isothermalizer. Dineen U.S. Pat. No. 3,220,178 used meshed fins in an air engine built for the U.S. Army.
Although the isothermalizer idea is old, the reasons it has not been more popular in Stirling engine design are:
1. It is usually more expensive to build. PA1 2. It is necessary only for machines operating over a small temperature difference. PA1 3. Good performance can be realized without it.
The mainstream of Stirling engine development from the original engine has been by increasing the surface area of the flow-through heater, regenerator and cooler. Only small, low pressure Stirling engines such as are used in the artificial heart can reasonably employ a single annulus to act as a heater, regenerator and cooler in different parts of the annulus. Larger Stirling engines regularly use fins or tube bundles in the heater and cooler and stacked screens or knitted steel wire in the regenerator. It is easy to design the engine heat exchangers so that heat transfer is very good, but flow friction is so high that all the power is consumed in internal flow friction. It is also easy to design the heat exchangers with negligible flow friction but with inadequate heat transfer. Furthermore, one is not free to build big heat exchangers with adequate heat transfer and acceptable flow loss as is possible in steam engines or in gas turbines. In these machines extra large heat exchangers add to the cost and marginally improve power output and efficiency. On the other hand, in a Stirling engine extra large heat exchangers add to the cost and may increase or decrease efficiency, but always greatly reduces the power output. This effect is the inevitable result of not having valves or pumps as are used in Rankine or Brayton cycle machines. Dead volume in a Stirling machine decreases the pressure change that is possible for a given volumetric displacement and therefore reduces power output capability.
Dead volume is always needed for the regenerator. A regenerator is needed for good efficiency. Some type of matrix is needed with low longitudinal thermal conductivity. During half the cycle heat is being transferred into the matrix at each point. During half the cycle heat is being transferred back out. The matrix must have adequate heat capacity so that its temperature does not change appreciably during a cycle. There must be a large surface for heat transfer so that at each point as the gas moves through the matrix only a small temperature difference exists. The flow area must be large enough so that flow resistance is small without heat conduction becoming too large.
For the usual flow-through type of heater and cooler, dead volume is also needed. For flame heating the size of the heater is controlled by the flame-side heat transfer area. A heat pipe heated gas heater is better because it is much smaller because the working gas side surface is controlling. Tubes are usually used because the heat fluxes are high and temperature drop through the wall is manageable. Fins are cheaper but the temperature drop along the fins must not be neglected. Gas coolers are usually made from a very large number of tiny tubes with the water in cross flow. The cooler, and especially the heater, are costly parts of the Stirling engine. A number of concepts are now being tested to simplify the design and reduce the cost of the materials. The subject of this disclosure is an entirely new way to build the gas heater and cooler which appears simpler and cheaper than present methods. This new way lends itself to very large flow areas, which makes it possible to use air as a working fluid with little penalty.